refactor: consolidate metrics-server addon spec

[jackfrancis]

Reason for Change:

This PR consolidates the maintenance of the metrics-server addon into a single go template-aware YAML file.

Issue Fixed:

Addresses #3188

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #3211 into master will increase coverage by 0.07%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master    #3211      +/-   ##
==========================================
+ Coverage   71.37%   71.45%   +0.07%     
==========================================
  Files         147      147              
  Lines       25687    25639      -48     
==========================================
- Hits        18335    18320      -15     
+ Misses       6207     6177      -30     
+ Partials     1145     1142       -3     

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	39.64% <77.77%> (+0.01%)	⬆️
pkg/api/addons.go	97.88% <100.00%> (+<0.01%)	⬆️
pkg/engine/engine.go	85.95% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2dd276e...9040eac. Read the comment docs.

[mboersma]

/lgtm

Just a couple of non-binding, spitballing, raise-the-flag-and-see-who-salutes kind of comments.

[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [jackfrancis,mboersma]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment